Phosphoric acid (H3PO4) is commonly used in semiconductor processing for a variety of applications including wet etching and stripping of metals and nitrides. For example, in a conventional process for forming a gate stack structure, phosphoric acid is used to strip an inorganic anti-reflective coating (IARC) from the top of the polysilicon gate region. Phosphoric acid is also used to etch a silicon nitride hardmask during a conventional process for forming a shallow trench isolation structure.
Problems arise, however, because phosphoric acid also etches exposed silicon leading to roughening of the silicon surface. Conventional methods for protecting against attack by phosphoric acid include growth of a thermal oxide. Growth of a thermal oxide, however, adds processing complexity and exposes the semiconductor structure to the potentially damaging effects of high temperature processing. Furthermore, controlling the growth of thermal oxide thickness is difficult for thin films, and growing of thick thermal oxide films consumes large amounts of silicon and alters critical dimensions (CD).
Another conventional method for protecting exposed silicon against attack by phosphoric acid uses a protective film. The protective film, however, is deposited on all semiconductor structure surfaces, not just exposed silicon. As such, processing complexity is increased because the protective film requires subsequent removal.
Thus, there is a need to overcome these and other problems of the prior art and to provide a method to protect silicon from phosphoric acid.